The Leishmania parasite causes human disease with clinical symptoms ranging from-self healing cutaneous lesions to a fatal visceral infection. Additionally, in endemic areas, people infected with HIV are especially prone to Leishmania infection. The lack of understanding of cell biology and pathogenic mechanisms of this parasite makes the task of controlling this grave, worldwide health risk difficult. Closer to home it is particularly of concern to U.S. military personnel, their families and other travellers visiting or living in the endemic areas. To find novel methods for control of this pathogen,we have initiated study to understand the mechanism of parasite differentiation from avirulent(promastigote)to virulent(amastigote)form. This transformation is controlled by the genetic changes which take place in the parasite. We have developed a culture system which mimics the differentiation of Leishmania parasite from promastigotes to amastigotes. We rationalized that the genetic changes should proceed the morphological changes, therefore we made attempted to identify genes that are expressed immediately after the parasite differentiation is initiated, and are described as immediate early genes. To identify immediate early genes, RNA samples were collected before and at time intervals (1, 2, 4, 8, 16, 24, and 48 hrs) after induction of differentiation from promastigotes and amastigotes. These RNAs were first characterized by hybridization with three previuosly identified genes whose expression is differentially regulated between the pro-and the amastigotes. The results suggested that genes which were turned off over the course of differentiation did so rapidly while the genes that are up-regulated, the kinetics of up-regulation is slower. The differential kinetics of expression of these genes indicates that these RNA samples can be used to generate probes to screen libraries for novel genes which are expressed immediately after the initiation of differentiation. These novel genes may provide targests to block parasite differentiation and be useful for attenuated parasite vaccines.